Conventionally, it has been the practice to heat treat particulate bulk materials, for instance, in the manufacture of cement or in a process for calcining bauxite, by using oil as the fuel. In processes requiring high temperatures, above 2000.degree. F., it has been impractical to use conventional resistance heating elements because the life of such elements becomes very limited when they are heated beyond about 2000.degree. F.
The treating of alumina hydrate to recover aluminum from it involves the pre-heating of the hydrate to drive off water vapor and produce gamma phase alumina. The latter is not a stable product because it will absorb moisture again and return to the hydrate state. However, further heating of the gamma phase to 3000.degree. F. will convert the gamma phase alumina to alpha phase alumina which will not re-hydrate. Moreover, this heat treating reduces the bulk to about one-third of the bulk of the alumina hydrate and, therefore, to a good state for shipping to a refining plant. The present heat treating apparatus is designed to effect this conversion using microwave techniques.
A number of patents exist teaching the use of microwave heating in industrial processes and apparatus for treating lossy materials such as food, rubber, lumber, etc., and some patents show the use of microwave units for heating granular bulk materials, particularly where the flow of the materials need not be continuous, or if continuous, where the temperatures to be achieved are not very high as shown, for example, in U.S. Pat. No. 2,467,230 to Revercomb and U.S. Pat. No. 2,500,752 to Hanson. U.S. Pat. No. 3,422,242 to Miyata shows a semicontinuous unit in which the materials are introduced in steps separated by the opening and closing of oven doors, the patent also showing the use of water filled chokes to seal the doors when closed.
U.S. Pat. No. 3,469,053 to Levinson shows the use of thermal insulation inside a microwave oven to help retain the heat against leakage from the oven, and the same is true of U.S. Pat. No. 3,701,872 to Levinson, but these patents do not show the continuous flow of particulate bulk materials which are at the same time treated at very high temperatures, i.e., at about 3000.degree. F. as in the present invention. U.S. Pat. No. 3,678,238 to Yasouka shows a microwave choke sealing an oven door.